The development of production processes based on renewable resources is highly desired, for example for the generation of ethanol from cellulosic and lignocellulosic materials.
Cellulose material in pure form or in combination with hemicellulose and/or lignin is a valuable and readily available raw material for the production of chemicals and fuels. A key step in processing cellulose and lignocellulose is the hydrolysis of the beta-1,4-linked glucose polymer cellulose and the subsequent release of glucose monomers and short glucose oligomers such as cellobiose, cellotriose, etc. Enzymes that catalyze this reaction are found in various organisms, especially filamentous fungi and bacteria, that are capable of degrading and hydrolysing cellulose.
Continuous processes for converting solid lignocellulosic biomass into combustible fuel products are known. Treatment to make cellulosic substrates more susceptible to enzymatic degradation comprises milling, chemical processing and/or hydrothermal processing. Examples are wet oxidation and/or steam explosion. Such treatments increase the accessibility of cellulose fibers and separate them from hemicellulose and lignin.
A number of enzyme mixtures for hydrolysis of treated biomass are known in the literature. Typically a mixture of endoglucanase, exoglucanase and beta-glucosidase enzymes are required for the degradation of cellulose polymers. Among these cellobiohydrolase (CBH) enzymes, and more specifically cellobiohydrolase I (CBHI) enzymes, play a key role in the hydrolysis step as they provide the most processive enzymatic activity. CBHI enzymes catalyze the progressive hydrolytic release of cellobiose from the reducing end of the cellulose polymers. (Lynd L R, Weimer P J, van Zyl W H, Pretorius I S. Microbial cellulose utilization: fundamentals and biotechnology. Microbiol Mol Biol Rev. 2002 September; 66(3):506-77).
Hydrolyzed cellulosic materials contain several valuable carbohydrate molecules which can be isolated from the mixtures. Sugar containing hydrolysates of cellulosic materials can be used for microbial production of a variety of fine chemicals or biopolymers, such as organic acids, ethanol or higher alcohols (also diols or polyols) or polyhydroxyalkanoates (PHAs). One of the major uses of the sugar hydrolysates is in the production of biofuels.
Kurabi et al. (2005) describes preparations of cellulases from Trichoderma reesei and other fungi, such as Penicillium sp. The performance has been analysed on steam-exploded and ethanol organosolv-pretreated Douglas-fir. Better performance of enzyme mixtures appears to be a result of improved properties of single component enzymes as well as the effect of each compound in the mixture, especially the presence of beta-glucosidase. (Kurabi A, Berlin A, Gilkes N, Kilburn D, Bura R, Robinson J, Markov A, Skomarovsky A, Gusakov A, Okunev O, Sinitsyn A, Gregg D, Xie D, Saddler J. (2005) Enzymatic hydrolysis of steam-exploded and ethanol organosolv-pretreated Douglas-Fir by novel and commercial fungal cellulases. Appl Biochem Biotechnol. 121-124: 219-30).
Cellobiohydrolase sequences of the glucohydrolase class 7 (cel7) are known to the art from several fungal sources. The Talaromyces emersonii Cel7 cellobiohydrolase is known and expression was reported in Escherichia coli (Grassick A, Murray P G, Thompson R, Collins C M, Byrnes L, Birrane G, Higgins T M, Tuohy M G. Three-dimensional structure of a thermostable native cellobiohydrolase, CBH IB, and molecular characterization of the cel7 gene from the filamentous fungus, Talaromyces emersonii. Eur J Biochem, 2004 November; 271(22):4495-506) and Saccharomyces cerevisiae (Voutilainen S P, Murray P G, Tuohy M G, Koivula A. Expression of Talaromyces emersonii cellobiohydrolase Cel7A in Saccharomyces cerevisiae and rational mutagenesis to improve its thermostability and activity. Protein Eng Des Sel. 2010 February; 23(2):69-79), however the protein was either produced in inactive form or at rather low yields (less or equal to 5 mg/l). Hypocrea jecorina cellobiohydrolase I can be produced from wild type or engineered strains of the genus Hypocrea or Trichoderma at high yields. Improved sequences of Hypocrea jecorina Cel7A are disclosed by U.S. Pat. Nos. 7,459,299B2, 7,452,707B2. WO2005/030926. WO01/04284A1 or US2009/0162916 A1.
Positions leading to improvements were deduced from alignments with sequences from reported thermostable enzymes, suggested from structural information and shuffling of identified positions followed by limited screenings. Screening of larger libraries in transformable organisms such as Saccharomyces cerevisiae was reported by application of very sensitive fluorescent substrates, which resemble native substrates in a very restricted way. (Percival Zhang Y H, Himmel M E, Mielenz J R. Outlook for cellulase improvement: screening and selection strategies. Biotechnol Adv. 2006 September-October; 24(5):452-81).
The production of cellobiohydrolases from other fungal systems such as Thermoascus aurantiacus, Chrysosporium lucknowense or Phanerochaete chrysosporium was reported. Expression of Cel7 cellobiohydrolase from yeasts was reported, but enzymatic yields or enzyme properties remain unsatisfactory. (Penttilä M E, André L, Lehtovaara P, Bailey M, Teeri T T, Knowles J K. Efficient secretion of two fungal cellobiohydrolases by Saccharomyces cerevisiae; Gene. 1988; 63(1):103-12).
WO03/000941 discloses a number of CBHs and their corresponding gene sequences. Physiological properties and applications however were not disclosed. The fusion of cellulose binding domains to catalytic subunits of cellobiohydrolases is reported to improve the hydrolytic properties of proteins without a native domain.
US 2009042266 (A1) discloses fusions of Thermoascus aurantiacus Cel7A with cellulose binding domains from cellobiohydrolase I from Chaetomium thermophilum and Hypocrea jecorina. 
U.S. Pat. No. 5,686,593 reports the fusion of specially designed linker regions and binding domains to cellobiohydrolases.
Hong et al. (2003) describe the production of Thermoascus aurantiacus CBHI in yeast and its characterization. (Hong J, Tamaki H, Yamamoto K, Kumagai H Cloning of a gene encoding thermostable cellobiohydrolase from Thermoascus aurantiacus and its expression in yeast. Appl Microbiol Biotechnol. 2003 November; 63(1):42-50).
Tuohy et al. (2002) report the expression and characterization of Talaromyces emersonii CBH. (Tuohy M G, Walsh D J, Murray P G, Claeyssens M, Cuffe M M, Savage A V, Coughlan M P.: Kinetic parameters and mode of action of the cellobiohydrolases produced by Talaromyces emersonii. Biochim Biophys Acta. 2002 Apr. 29; 1596(2):366-80).
Nevoigt et al. (2008) reports on the expression of cellulolytic enzymes in yeasts. (Nevoigt E. Progress in metabolic engineering of Saccharomyces cerevisiae. Microbiol Mol Biol Rev. 2008 September; 72(3):379-412).
Fujita et al. (2004) reports on a Saccharomyces cervisiae strain expressing a combination of an endoglucanase, a beta glucosidase and a CBHII displayed on the cell surface. Cellobiohydrolase I (Cel7) was not used in this setup. (Fujita Y, Ito J, Ueda M, Fukuda H, Kondo A. Synergistic saccharification, and direct fermentation to ethanol, of amorphous cellulose by use of an engineered yeast strain codisplaying three types of cellulolytic enzyme. Appl Environ Microbiol. 2004 February; 70(2):1207-12).
Boer H et al. (2000) describes the expression of GH7 classified enzymes in different yeast hosts but expressed protein levels were low. (Boer H, Teeri T T, Koivula A. Characterization of Trichoderma reesei cellobiohydrolase Cel7A secreted from Pichia pastoris using two different promoters. Biotechnol Bioeng. 2000 Sep. 5; 69(5):486-94).
Godbole et al (1999) and Hong et al (2003) found that proteins of this enzyme class expressed fom yeast were often misfolded, hyperglycosylated and hydrolytic capabilities decreased compared to the protein expressed from the homologous host. (Godbole S, Decker S R, Nieves R A, Adney W S, Vinzant T B, Baker J O, Thomas S R, Himmel M E. Cloning and expression of Trichoderma reesei cellobiohydrolase I in Pichia pastoris. Biotechnol Prog. 1999 September-October; 15(5):828-33).
Kanokratana et al (2008), Li at al (2009) as well as CN01757710 describe the efficient expression of Cel7 CBH I enzymes, however these proteins are lacking cellulose binding domains required for efficient substrate processing. (Kanokratana P, Chantasingh D, Champreda V, Tanapongpipat S, Pootanakit K, Eurwilaichitr L Identification and expression of cellobiohydrolase (CBHI) gene from an endophytic fungus, Fusicoccum sp. (BCC4124) in Pichia pastoris. LProtein Expr Purif. 2008 March; 58(1):148-53. Epub 2007 Sep. 19; Li Y L, Li H, Li A N, Li D C. Cloning of a gene encoding thermostable cellobiohydrolase from the thermophilic fungus Chaetomium thermophilum and its expression in Pichia pastoris. J Appl Microbiol. 2009 June; 106(6):1867-75).
Voutilainen (2008) and Viikari (2007) disclose Cel7 enzymes comprising thermostable cellobiohydrolases, however with only low to moderate expression levels from Trichoderma reesei. (Voutilainen S P, Puranen T, Siika-Aho M, Lappalainen A, Alapuranen M, Kallio J, Hooman S, Viikari L, Vehmaanperä J, Koivula A. Cloning, expression, and characterization of novel thermostable family 7 cellobiohydrolases. Biotechnol Bioeng. 2008 Oct. 15; 101(3):515-28. PubMed PMID: 18512263; Viikari L, Alapuranen M, Puranen T, Vehmaanperä J, Siika-Aho M. Thermostable enzymes in lignocellulose hydrolysis. Adv Biochem Eng Biotechnol. 2007; 108:121-45).
Grassick et al. (2004) disclose unfolded expression of Cellobiohydrolase I from Talaromyces emersonii in Escherichia coli but not in yeast. (Grassick A, Murray P G, Thompson R, Collins C M, Byrnes L, Birrane G, Higgins T M, Tuohy M G. Three-dimensional structure of a thermostable native cellobiohydrolase, CBH IB, and molecular characterization of the cel7 gene from the filamentous fungus, Talaromyces emersonii. Eur J Biochem. 2004 November; 271(22):4495-506).
Therefore, there is a need for cellulase enzymes with improved characteristics for the use in technical processes for cellulose hydrolysis. In particular there is a need for CBH enzymes with higher catalytic activity and/or higher stability under process conditions. Moreover there is a need for CBH enzymes with higher productivity in fungal and/or yeast expression and secretion systems.